Compact radiation detector

ABSTRACT

An apparatus for detecting radiation is provided. In embodiments, at least one sensor is disposed on a surface of a wafer-like substrate. At least one sensor medium is fixed relative to the substrate, optically or electrically coupled to the sensor, and separated from the sensor by no more than the substrate thickness. An electronic signal-processing circuit is connected to the sensor and configured to produce an output when the sensor is stimulated by a product of an interaction between the sensor medium and impinging radiation. The sensor is configured to collect, from the sensor medium, charge and/or light produced within the sensor medium by interactions with impinging radiation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 13/559,370, filed on Jul. 26, 2012 by M.S. Derzon under thetitle “Ion Chamber Based Neutron Detectors,” which application iscommonly owned herewith and the entirety of which is incorporated hereinby reference. The aforesaid U.S. patent application Ser. No. 13/559,370is a Continuation-in-Part of U.S. patent application Ser. No. 12/046,041filed Mar. 11, 2008 and now abandoned, which claims the benefit of U.S.Provisional Patent Application Serial No. 60/894,700, filed Mar. 14,2007.

The subject matter of this application is related to the subject matterof U.S. patent application Ser. No. 14/192,688, filed on common datewith the filing hereof by M.S. Derzon et al. under the title“Multiple-Mode Radiation Detector,” and issued on Aug. 25, 2015 as U.S.Patent No. 9,116,249, which patent is commonly owned herewith.

GOVERNMENT CONTRACT REFERENCE

This invention was developed under Contract DE-AC04-94AL85000 betweenSandia Corporation and the U.S. Department of Energy. The U.S.Government has certain rights in this invention.

TECHNICAL FIELD

The present invention is directed generally to radiation detectors, and,more particularly, to detectors for neutrons and gamma rays or x-rays.The invention also has potential applications for the detection of alphaand beta radiation, which therefore should not be excluded from itsscope.

BACKGROUND OF THE INVENTION

Various tradeoffs exist in the design of radiation sensors. Some of thedesirable attributes of a radiation sensor that are affected bytradeoffs include cost, sensitivity, spectral discrimination,discrimination of one particle type from another, spatial discriminationor imaging ability, particle tracking, and energy consumption. Althoughradiation sensors have been in development for many years, there remainsa need for new designs that achieve favorable tradeoffs among these andother attributes. Two rapidly developing fields that have greatlyincreased the demand for new sensor designs are radiologicalsurveillance and radiological medicine.

One recent design approach, motivated in particular by the need for newsensors for radiological surveillance, is described in our U.S. patentapplication Ser. No. 13/559,370, cited above. Embodiments describedthere are neutron detectors using, e.g., helium-3 as a sensing medium.Features are described that improve the rejection of gamma-ray-inducedbackground noise, reduce the total helium-3 requirement, and provide forincreased spatial resolution and directional discrimination.

One of the embodiments we described was a thermal neutron detector inwhich a high pressure ion chamber was formed in a dielectric material.First and second electrodes were formed in the high pressure ionchamber, which was filled with a neutron absorbing material to chamberpressures up to several hundred atm. and was surrounded at least in partby a neutron moderating material. The high pressure ion chamber had apair of parallel, substantially planar surfaces on which respectivefirst and second electrodes were formed.

Another of the embodiments we described was a neutron detector withmonolithically integrated readout circuitry. In that embodiment, the ionchamber was formed in a bonded semiconductor die that included an etchedsemiconductor substrate bonded to an active semiconductor substrate. Thefirst and second electrodes were formed in the ion chamber and wereelectrically coupled to readout circuitry formed in a portion of theactive semiconductor substrate.

Although useful, the above-described design approach still leaves roomfor extensions, e.g. for alpha, beta, and gamma particle detection, andfor competing approaches that may provide further capabilities or thatmay be more optimal for certain applications.

SUMMARY OF THE INVENTION

Our invention in one aspect is an apparatus for detecting radiation. Inembodiments, at least one sensor is disposed on a surface of awafer-like substrate. At least one sensor medium is fixed relative tothe substrate, optically or electrically coupled to the sensor, andseparated from the sensor by no more than the substrate thickness. Anelectronic signal-processing circuit is connected to the sensor andconfigured to produce an output when the sensor is stimulated by aproduct of an interaction between the sensor medium and impingingradiation.

The sensor is configured to collect, from the sensor medium, chargeand/or light produced within the sensor medium by interactions withimpinging radiation. In some embodiments, at least one sensor is aphotodiode configured to collect light from the sensor medium. In someembodiments, at least one sensor is an electrode arrangement configuredto collect charge generated in and drifted through the sensor medium.The collected charge may, e.g., be primary charge generated directly byimpinging radioactive particles, or it may also include, e.g., secondarycharges generated by interactions between the primary charges and thesame sensor medium.

In embodiments, a plurality of sensors are disposed on the substratesurface and the sensor medium is disposed within a laterally extendingarray of cavities, each of which is aligned with at least one respectivesensor. In some embodiments, the substrate is an SOI substratecomprising a device layer and a handle layer, the sensors are disposedon the device layer, and the cavities are formed in the handle layer. Inother embodiments, the sensors are disposed on a silicon or SOIsubstrate, the cavities are defined between said substrate and a furthersilicon or SOI substrate, and the two substrates are bonded together.

In embodiments, the signal-processing circuit comprises a respectivepreamplifier for each said sensor; and said preamplifiers are separatedfrom their respective sensors by no more than the substrate thickness.

In embodiments, a rectangular array of sensors is fixed in an integralunit with the sensor medium, and wherein said integral unit is one oftwo or more similar units arranged in a vertical stack. In embodiments,at least one motherboard containing processing circuitry is connected toone or more output ports on each of the respective integral units. Inembodiments, each said integral unit includes an electronic circuitconfigured to provide signal output that comprises two-dimensional imageinformation, and wherein at least one motherboard includes an electroniccircuit configured to process the two-dimensional information from theintegral units so as to produce output that comprises three-dimensionalimage information.

In embodiments, two or more sensor media are arranged such that at leasta first sensor medium is separated from the sensor by no more than thesubstrate thickness and at least a second sensor medium is separatedfrom the sensor by a greater distance than the first sensor medium. Inembodiments, sensor media are disposed in a stack of layers such that atleast some impinging radioactive particles can penetrate one or morepreceding layers before stopping in a layer of the stack. Inembodiments, the electronic signal-processing circuit is configured todiscriminate radioactive particles based on the number of sensor-mediumlayers that are penetrated.

In embodiments, two or more sensors are configured to respond todifferent products of interactions between the sensor medium andimpinging radiation, and the electronic signal-processing circuit isconfigured to further discriminate radioactive particles based ondifferences in the interaction products.

In embodiments, the wafer-like substrate comprises an SOI substratehaving a silicon layer and a silicon oxide layer; the sensor medium isdisposed within a cavity; the sensor comprises a pair of electrodespositioned on opposing sides of the cavity; the silicon oxide layerprovides electrical isolation between the pair of electrodes; and atleast one of the electrodes is wholly or partly defined as a portion ofthe silicon layer that has been doped to increase its electricalconductivity.

In another aspect, our invention is a radiation detection apparatus inwhich at least one sensor is disposed on a surface of a wafer-likesubstrate. At least one sensor medium cavity is fixed relative to thesubstrate and separated from the sensor by no more than the substratethickness. A duct arrangement is provided for filling the cavity with,and emptying the cavity of, an interchangeable sensor medium.

Any of various sensor media may be used in specific embodiments of theinvention. Exemplary sensor media include, without limitation, hydrogen,deuterium, helium, krypton, xenon, boron, cadmium telluride, cadmiumzinc telluride, thorium oxide, uranium oxide, propane, methane, andthallium bromide.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1A is a cut-away drawing in side view of an exemplaryion-chamber-based thermal neutron detector according to the presentinvention;

FIG. 1B is a top plan drawing of the exemplary ion chamber based thermalneutron detector of FIG. 2A;

FIG. 2A is a side cut-away drawing of an exemplary ion chamber basedneutron detector with monolithically integrated readout circuitryaccording to the present invention;

FIG. 2B is a top plan drawing of the exemplary ion chamber based neutrondetector of FIG. 3A;

FIG. 2C shows an embodiment in which respective sensor layers are formedin a silicon-on-insulator wafer in which the handle layer has cavitiesfilled with radiation-sensitive material and electronic circuitry isformed on the device layer.

FIG. 3A is a side cut-away drawing of an alternative exemplary ionchamber based neutron detector according to the present invention;

FIG. 3B is a top plan drawing of the alternative exemplary ion chamberbased neutron detector of FIG. 5A.

FIG. 4 provides a notional representation of a 3D detector array inwhich multiple layers, each containing a 2D array of silicon-basedpixels, are stacked together.

FIG. 5 provides a cross-sectional view of one illustrative layeredsensor that has been configured for containment of a liquid or gaseoussensing medium.

FIG. 6 provides a cross-sectional view of a stack of illustrativelayered sensors that include wafers of a semiconductiveradiation-sensitive medium.

FIG. 7 provides a view of a single detector substrate, containing a 2Darray of pixels, that has been configured to be stacked with similarsubstrates to form a 3D array.

FIGS. 8A and 8B provide a view of an assembly of stacked substrates ofthe kind shown in FIG. 7.

FIG. 9 provides a view of a bonded silicon substrate that has beenconfigured to contain one or more fluid-filled cavities corresponding torespective detector pixels, and further configured so that it can bestacked in an arrangement such as the arrangement of FIGS. 8A and 8B.

FIG. 10 provides a cross-sectional view of a vertically integratedbimodal detector arrangement.

FIG. 11 provides a top-down plan view of a horizontally integrated,two-dimensional, dual channel pixel array for bimodal radiationdetection.

FIG. 12 provides a cross-sectional view of a version of the sensordevice of FIG. 11 that has been adapted for a gaseous sensor medium. Thefigure also schematically depicts a detector event caused by thecollision of a particle of radiation in the sensor medium.

FIG. 13 provides a cross-sectional view of a version of the sensordevice of FIG. 11 that has been adapted for a solid sensor medium.

FIG. 14 provides a view of an arrangement in which two detector ASICs ofthe kind illustrated in FIG. 12 are joined together for greaterphoton-collection efficiency.

FIG. 15 provides a high-level schematic representation of anillustrative on-board pre-amplifying and pulse-shaping circuit for apixel according to the invention in some embodiments.

DETAILED DESCRIPTION OF THE INVENTION

We will first briefly review the two abovesaid detector embodiments ofU.S. patent application Ser. No. 13/559,370. We will then describefurther ideas and developments and exemplary embodiments thereof.

It should be noted in this regard that although the embodimentsdescribed in U. S. patent application Ser. No. 13/559,370 are directedspecifically to neutron detection, they are specific implementations ofa more general idea for a charge-collecting device that in its variousimplementations can detect radioactive particles of various kinds,including gamma ray and x-ray photons, neutrons, and alpha and betaparticles, by collecting primary charges generated by the particleinteractions in the sensor medium. As understood in such a broad aspect,the sensor medium may have any of various compositions, includingwithout limitation low-atomic-number gases, noble gases, andsemiconductors. Indeed, in one broad aspect the invention may beregarded, conceptually, as a platform containing a sensor medium thatfor different embodiments can be changed to provide, within the samearchitectural framework, a scalable detection volume and variablecapabilities. As will be seen below, certain specific embodiments offerfeatures such as low-capacitance coupling to the electronic detectioncircuitry and charge-generation dynamics that do not require avalancheor gain mechanisms to produce detectable signals. These features canlead to improved resolution in the detection of particles having highenough specific energy deposition. Also, as will be seen below, theinvention in a broad aspect allows for dual-mode detection, in whichboth photons and electrons generated by the particle interactions arecollected and detected.

The first of the abovesaid detector embodiments is illustrated asdetector 200 of FIGS. 1A and 1B, which shows among other things highpressure ion chamber 202 filled with neutron absorbing material 204,respective bottom and top electrodes 206, 208 formed on opposingsurfaces of the ion chamber, and neutron moderating material 210 whichmay, for example, be polyethylene. The ion chamber is desirably formedof a dielectric material such as silicon or FR4 glass laminate. Theneutron absorbing material 204 may comprise one or more of variousmaterials such as helium-3, helium-4, xenon, hydrogen, propane, ormethane. For at least some applications, a combination of helium-3 andxenon may be preferable.

To improve the efficiency of the neutron absorbing material 204, wefound it desirable for distance 106′ between bottom electrode 206 andtop electrode 208 to be less than or equal to the 50% attenuation lengthfor thermal neutrons in neutron absorbing material 204 at the chamberpressure. For a similar reason, we found that the area of thecross-section of high pressure ion chamber 202 parallel to electrodes206, 208 is desirably greater than or equal to 100 times the square ofdistance 106. Reducing the distance between electrode 206 and electrode208 also advantageously increases detector sensitivity because itreduces the drift time before the radiation-induced charges arecollected and hence reduces the amount of charge that is lost torecombination.

The second of the abovesaid embodiments is illustrated as detector 300of FIGS. 2A and 2B. As seen in the figures, the detector includes an ionchamber formed in a bonded semiconductor die and further includesmonolithically integrated readout circuitry 312. The bondedsemiconductor die is formed by bonding etched semiconductor substrate302 to active semiconductor substrate 304 at bond points 306. Therespective bonded portions of the semiconductor die are exemplarilyformed of silicon or of silicon-based materials. In one exemplaryembodiment, etched semiconductor substrate 302 is formed of silicon andactive semiconductor substrate 304 is a silicon on insulator (SOI)substrate.

Monolithic integration of the readout circuitry may advantageouslyreduce the capacitance and hence the noise associated with readout ofthe charge collected from the reaction particle ion trails of detectedneutrons. The use of monolithically integrated readout circuitry canalso facilitate the design of a pixelated ion chamber based neutrondetector in which a two (or three) dimensional array of individualneutron detectors, such as detector 300, is deployed.

In the abovesaid patent application, we also described a process formaking the detector of FIGS. 2A and 2B. Very briefly, with furtherreference to the figures, a hollow space which will form the volume ofthe ion chamber is etched into substrate 302. Top electrode 308 isformed on the top surface of the hollow space. Monolithically integratedreadout circuitry 312 is formed in active semiconductor substrate 304using, e.g., a CMOS or other standard semiconductor fabrication process.Bottom electrode 310 is formed on the opposite surface of activesemiconductor substrate 304, which is desirably parallel to the topsurface of the hollow space of etched semiconductor substrate 302 afterbonding. Bottom electrode 310 may be electrically coupled tomonolithically integrated readout circuitry 312 during the fabricationprocess or in a subsequent processing step. Monolithically integratedreadout circuitry 312 is also electrically coupled to top electrode 308.

In the abovesaid patent application, we also described an ion chamberdesign in which an array of columns extended between the twosubstantially planar major surfaces of the chamber. The purpose of thearray of columns was to provide structural support so that the ionchamber could be extended to a relatively large cross-sectional areawhile still withstanding extremely high internal pressures due to thegas fill. FIGS. 3a and 3b illustrate such a design, in which chamber 500is formed by bonding etched die portion 502 to flat die portion 504 atbond points 506. Etched die portion 502 is etched to include the arrayof columns.

Those skilled in the art will understand from the foregoing descriptionthat by applying the principles described there, it will be possible tocreate a sensor in which a layer of radiation-sensitive material isjuxtaposed to a substantially parallel layer on which is disposedelectronic circuitry for detecting the response of the sensitivematerial to impinging radiation. Referring back to sensor 300 of FIGS.2A and 2B, for example, it will be understood that the ion chambersubstantially occupies a cavity etched in substrate 302, and that thereadout circuitry is disposed on substrate 304. The respectivesubstrates are substantially parallel and are bonded together. It willalso be appreciated that the readout circuitry is closely coupled, inthe geometrical sense, to the ion chamber, since it is separatedtherefrom by no more than the thickness of substrate 304.

In the following discussion, we will use the term “layered sensors” torefer generally to sensor architectures in which a layer ofradiation-sensitive material (also referred to herein as the “sensingmedium”) is juxtaposed to a substantially parallel layer on which isdisposed electronic circuitry for detecting the response of thesensitive material to impinging radiation, and in which the detectioncircuitry is separated from the radiation-sensitive material by no morethan a substrate thickness.

Some potential benefits of layered sensors that we have been exploringinclude the possible use of exotic semiconductor media such as uraniumdioxide and uranium metal, the use of gaseous or liquid media such ashelium gas and pressurized or liquefied xenon for selectable detectoroperation in ionization or proportional counter modes, the use ofadvanced sensor electronics offering low noise and the potential for awide range of timing and pulse-shape-analysis options, and customizablesensor geometries ranging from small to large total volumes.

One particular advantage that layered sensor architectures may offer isthe ability to subdivide the energy deposition volume into numerousspatially separated energy deposition regions. Each such region may beintegrated with a respective electronic element to define an individualpixel of a one-dimensional or two-dimensional pixel array, or to definean individual voxel of a three-dimensional voxel array. A great deal ofinformation about the incident radiation may be gained by collecting andprocessing information representing the respective amounts of energydeposited in the various array elements, together with the timing of theindividual energy-deposition events.

We have developed further approaches to the design of layered sensors,which we will now describe.

In one such approach as shown in FIG. 2C, the respective sensor layersare formed in a silicon-on-insulator (SOI) wafer. The handle layer 320is etched to form cavities 322 that are filled with theradiation-sensitive material, and the electronic circuitry 324 is formedon the device layer 326 (which is also sometimes referred to as the“epilayer” or the “SOI layer”). A typical SOI wafer useful for thispurpose would be six inches in diameter and 700 μm in total thickness.

In an alternative layered arrangement, a semiconductor substrate onwhich electrodes and detector circuitry have been formed is juxtaposedto an external layer of radiation sensitive material which may beformed, e.g., as a coating on a separate substrate or as an independentsheet of material.

In either type of arrangement, electrodes can be juxtaposed to thesensing medium either by mechanical placement or by lithographicpatterning of a substrate surface that faces the radiation-sensitivematerial.

Moreover, electrical contacts can be placed so as to facilitate thestacking of two-dimensional pixel arrays to form three-dimensional (3D)arrays. FIG. 4, for example, provides a notional representation of sucha 3D array, in which, illustratively, five layers 401-405, eachcontaining a 2D array of silicon-based pixels, are stacked together andsupported by a motherboard 410, to which they are electrically connectedby wirebonds 420 between the motherboard and contact pads at the edgesof the respective layers. Within each layer, the pixels may be formed onthe surface of a common wafer which also contains on-board processingelectronics for that layer. Alternatively, the respective pixels may beformed on individual silicon chips that are bonded, exemplarily byflip-chip attachment, to a common substrate that likewise may containon-board processing electronics.

FIG. 5 provides a cross-sectional view of one illustrative layeredsensor that has been configured for containment of a liquid or gaseoussensing medium. As seen in the figure, a pair of high-resistivitysilicon cover layers 520, 521 are spaced apart and supported by aninsulating spacer 525 of, e.g., annular conformation that is containedbetween them, so as to define a cavity 530 to be filled with the fluidsensing medium. A pair of outer plates 540, 541 provide mechanicalprotection and shielding, if required. Each cover layer has a pair ofinner and outer metallized regions 550, 551 that provide electrodes orelectrical contacts, and that are joined together by a metal feedthrough555, as was required for electrical connectivity because of the highresistivity of the silicon layers. A threaded aperture 560 in one of theouter plates, together with a concentric aperture 561 in the adjacentcover layer, provides an inlet for filling the cavity with the sensingmedium. A fitting 570, such as a bulkhead gas fitting, may be mounted inthe apertured outer plate. A fill tube (not shown), such as a1/16^(th)-inch stainless steel gas capillary, may be mated to thefitting. In a prototype that we constructed and tested, the detectorelectronic circuitry was placed in a discrete external package.

FIG. 6 provides a cross-sectional view of a stack of illustrativelayered sensors. Each stage in the stack of FIG. 6 is similar to oneinstantiation of the sensor of FIG. 5. As shown in the figure, wafers600 of a semiconductive radiation-sensitive medium are provided. Onesuch medium is thallium bromide (TlBr), which is a semiconductor with abandgap somewhat over 4 eV and which is a known scintillator useful fordetection of, among other things, gamma and neutron radiation. In thepresent context, the TlBr is electrically biased at, e.g., 200 V todeplete it of charge carriers and used as a charge-generation detector;that is, neutron-absorption and gamma-absorption events are detectedbased on the electron-hole pairs that are generated and collected onelectrodes within the cavity. Optionally, photodiodes may be included inelectronics layers so that scintillation produced by the absorptionevents can also be detected.

It should be noted in this regard that the oxides of uranium and thoriummay also be useful as semiconductive sensor media. Although thesematerials are radioactive and hence contribute background noise due totheir own radioactive emissions, there are techniques for processing anddiscriminating the background signal. This may be desirable because,aside from the background noise, these materials have very desirablecharacteristics for pulsed detection, such as is encountered in medicalapplications, monitoring the health of machinery, nuclear materialssurveillance, and oil well logging.

Other radiation-sensitive semiconductors that may be useful as sensormedia include cadmium telluride and cadmium zinc telluride.

By way of example, a non-linear filter of the kind designed for theremoval of salt-and-pepper noise from images can be used to recognizeand cancel the alpha-particle background due, e.g., to the radioactivedecay of depleted uranium. The salt-and-pepper effect is due in thisinstance to the random pattern of detection spikes, e.g. very brightscintillations, caused by the alpha particles. We note in this regardthat a kilogram of depleted uranium would provide very roughly onemillion decays per second. If distributed over a 1000×1000 array ofpixels, this amounts to only one decay per pixel per second, on average.The processing of such a low level of activity, corresponding to oneHertz of background noise, would be well within the capabilities ofcurrent filter technology.

In the figure, which of course should be understood as merely exemplaryand not limiting, six TlBr wafers 600 are shown in a stack. The middlefour wafers form two pairs 610, 611, within each of which two wafers arearranged so that backside metallization layers 620 formed on each of thewafers are pressed together and bonded to each other. The stack may becontinued by similarly bonding the outermost wafers to further wafersnot shown in the figure.

With further reference to the figure, it will be seen that each wafer isbonded to an electronics substrate which may, e.g., be a double-sided ormultilayer flex circuit. In our terminology, the “backside” of the waferis the side that is bonded (if at all) to another wafer, and the “frontside” is the side bonded to an electronics substrate. In the exampleshown in the figure, the front side of each wafer is metallized,patterned, and provided with solder bumps 634 that may be embedded in anunderfill layer 635, i.e., an insulative layer used, among other things,for pixel isolation. A substrate 640, e.g. a flex circuit, populatedwith electronics circuitry specific to that wafer is bonded to the waferusing the solder bumps. The electronics substrates belonging to twoadjacent wafers (i.e., “outer substrates”) are mutually bonded, fromopposite sides, to a further substrate 650 (i.e., an “inner substrate”)that has a protruding edge (not shown in the figure) bearing solder padsor other electrical contact pads for external connection, and that mayoptionally include further processing circuitry. Electrically conductivevias formed in the outer substrates establish electrical connectivitybetween the wafer on one side of the outer substrate to the innersubstrate on its other side.

With still further reference to the figure, it will be seen that thebonded backsides of the wafers are interconnected with wirebonds 660 andenergized to, e.g., the 200 V level used for biasing the wafers.

For the fabrication of a pixellated array, the electronic circuits maybe formed according to known lithographic processing, such as CMOSprocessing, on one or more silicon or SOI wafers. Solder bumps are addedto the wafers, after which they are diced to form individual chips, eachcorresponding to one pixel. The chips are then bonded, as appropriate,to blocks of the radiation-sensitive medium and to the inner substrates.

Exemplary dimensions for certain features of the arrangement of FIG. 6are: chip edge length, one inch; chip thickness, 8 μm; solder bumpdiameter, in the range 8-35 μm; inner substrate thickness, 68μm; sensingmedium thickness, 200 μm.

One alternative semiconductor material that may be substituted in placeof TlBr in this context includes boron nitride, which is useful forthermal neutron detection.

FIG. 7 provides a view of another implementation, in which a singlesubstrate 700, containing a 2D array 710 of pixels (not individuallyshown), has been configured to be stacked with similar substrates toform a 3D array. The substrate of FIG. 7 is a silicon or SOI substratein which a border area 720 has been provided with through-silicon vias(TSVs) 730 to provide electrical connectivity from one side of thesubstrate to the other.

FIG. 8a provides a view of an assembly of, e.g., ten stacked substrates800 of the kind shown in FIG. 7. Such an assembly can be bonded togetherand tested. After testing confirms that they operate as specified, aplurality of such assemblies 805 can be bonded to a motherboard 810 asshown in FIG. 8b . Assembly-to-assembly alignment in the verticaldirection (as seen in the figure) can be facilitated by precision formedslots 820 on the motherboard wafer.

As seen in FIG. 8b , vertical elements of the motherboard 810 may bepaired in order to provide support on two opposing sides of thehorizontally-extending (as seen in the figure) substrates. The wideboards 830 seen fitted within slots in the motherboard at the bottom ofeach stack of ten wafers are useful for cooling. The stack makesexternal connections via a pattern of pinouts from, e.g., the TSVsdepicted in the border area of FIG. 7.

In a pixellated arrangement, each sensor, e.g. each charge-collectingelectrode arrangement or each photodiode, advantageously has its ownon-board preamplifier, as well as possibly other dedicated circuitry.

In alternative arrangements, a row of pixels may be interrogated bystripline readout, in which pulse timing is related to sequential pixelposition. In such arrangements, a single preamplifier may serve anentire row of pixels.

As noted, one advantage of our layered detector architecture is that atleast the first stage of the signal-processing electronics can bebrought very close to the sensor medium. Advantageously, this firststage includes the first-stage preamplifier and the timing electronics.By way of example, circuitry of that kind is readily implemented in thedevice layer of an SOI wafer. One advantage of such closelygeometrically coupled circuitry is an improvement in sensitivity. Itshould be noted in this regard, however, that silicon and SOI wafers arenot the only potentially useful electronics substrates. Other substratematerials potentially useful for that purpose include silicon nitride,silicon carbide, polyimide, and various other polymeric materialsuseful, e.g., as substrate materials in flex circuits.

As noted, each detector substrate can include on-board circuitry. For apixellated array, the on-board circuitry can include a processorconfigured to output two-dimensional image information. If, e.g., thesubstrate is part of a three-dimensional stack as discussed above, amotherboard (generally to be understood as encompassing any substratethat is orthogonal to the substrates of the stack) can further include aprocessor that accepts the two-dimensional information from the varioussubstrates and provides three-dimensional image information as output.Such three-dimensional information may include, for example, spatiallyand temporally resolved information about individual radiation-inducedevents, and information that tracks the trajectories of individualradioactive particles as they penetrate the stack.

On-board circuitry on an individual substrate or on a motherboard canalso include a wireless data-transfer port to facilitate read-out to asmartphone or other wireless terminal.

Another advantage of our layered architecture is that, at least when agaseous sensor medium is used, it offers the possibility to selectivelyoperate in either an ionization chamber mode or a proportional detectormode. Which mode a particular detector operates in depends on thecomposition and pressure of the gaseous medium, the electrode geometry,and the operating voltage. Ionization chambers operate in a lowervoltage range than proportional detectors. In general, ionizationchambers are more sensitive than proportional detectors, but unlikeproportional detectors, they are unable to measure particle energy. Webelieve that layered detectors can be designed, in which the same devicecan be operated in either mode depending on the gas fill and theoperating voltage. With judicious electrode design, we believe it mayeven be possible to select between modes based solely on the operatingvoltage.

Another advantage of the layered architecture, which we will discuss ingreater detail below, is that it readily lends itself to bimodal, ormultimodal, detection schemes in which at the sensor level, differentsensor media, having complementary properties, are clustered together,and in which at the detection level, charge-collection circuitry andphotonic circuitry can cooperate in a complementary manner.

Turning now to FIG. 9, shown there is a view of a bonded siliconsubstrate that has been configured to contain one or more fluid-filledcavities corresponding to respective detector pixels, and furtherconfigured so that it can be stacked in an arrangement such as thearrangement of FIGS. 8a and 8b . To form the detector substrate of FIG.9, two SOI wafers 901, 902 are provided, each including alow-resistivity silicon device layer 903, 904, a high-resistivitysilicon handle layer 905, 906, and between the silicon layers, a buriedoxide layer 907, 908. The resistivity of the various portions of the SOIwafers may be controlled by ion implantation, for example.

Electrodes 910, 911 are provided on opposing silicon wafer surfacesdistal the cavity or cavities 915.

In the view of FIG. 9, the two SOI wafers are shown separated by apre-assembly gap 920. The two wafers will subsequently be bondedtogether to form the finished article. It should be noted in this regardthat although both the upper and the lower wafer have been etched toform respective half-cells, there may be applications for which it issufficient to etch only one of the two wafers to form the cavity for thesensor medium. It should also be noted that the cavity depth is a designvariable that may be adapted, e.g., to the penetration depth of theparticular chosen sensor medium.

In the implementation that is shown in the figure, the handle layer ofeach SOI wafer is etched to about one-half the desired cavity depth. Therespective handle layers are mated to each other to form the fullcavities. Prior to assembling the wafers to each other, a thermal oxidelayer is formed on each of the mating surfaces. Upon assembly andthermal bonding, the thermal oxide forms the bond between the two SOIwafers.

To facilitate the filling of the cavities with sensor fluid, thecavities are interconnected by etched fill channels 930. Inlet andoutlet holes (not shown) for coupling to intake and exhaust manifolds(not shown) are etched into one or both of the SOI wafers.

One advantage of structures such as those shown here is that thedetector can be used with interchangeable sensor media. That is, ductssuch as the inlet and outlet holes mentioned above can be used to fillthe cavities with any of various alternative fluid sensor media, and caneven be used to later transfer the selected medium out of the cavitiesand replace it with a different sensor medium.

The low-resistivity silicon layers serve as the cavity electrodes. Webelieve it will not be necessary to dice the silicon for this purpose,because the separation between individual electrodes is large relativeto the wafer thickness. Because the silicon is relatively resistive, therespective electric fields in such a geometry will remain highlylocalized, so there will be individual charge collection by the separateelectrodes.

We note that in an arrangement in which a pair of electrodes are placedat opposing sides of a sensor medium, it is advantageous for theelectrode separation to be less than a charge-carrier absorption depthof the medium.

It should also be noted in this regard that silicon may be replaced byalternative substrate materials such as glass or polymeric materialsaccording to know techniques for the processing of electronic sheetgoods. In some implementations, moreover, liquid-phase epitaxy might beuseful for filling the cells with sensor medium.

In an exemplary assembly of the kind described above, thehigh-resistivity silicon layers are 125 μm thick, the low-resistivitysilicon layers are 500 μm thick, the oxide layers are 1-2 μm thick, andthe electrode diameter is 9 mm. The fill channels are exemplarily 500 μmwide and as deep as the cavities. Etched columns (not shown) areoptionally provided between the upper and lower silicon layers forsupport to strengthen the gas cavities for large-area cells at very highgas pressures.

As shown in the figure, signal-processing circuitry is provided,exemplarily using conventional CMOS fabrication processes, on thesurface of one or both device layers. Photodetectors, such asphotodiodes, may be incorporated in the circuitry for detection of,e.g., scintillation events.

FIG. 10 provides a cross-sectional view of a detector arrangement thatis vertically integrated, i.e., there are multiple (in this case, two)sensor layers that are stacked in the direction of incident radiation.As seen in the figure, the radiation 1000 to be detected is incidentfrom the bottom of the figure, and a detector 1010 for longer-rangeradiation, such as neutron and gamma radiation, is stacked above adetector 1011 for shorter-range radiation such as alpha and betaradiation and soft x-radiation.

Each sensor layer includes a sensor medium 1020 that scintillates in amanner that at least partially discriminates the type of particle thatis detected. Two such scintillators 1021, 1022 are shown schematicallyin the figure in each sensor layer. To make them visible in the figure,the scintillators have been rotated from their actual, horizontaldisposition.

Such scintillator materials are known. For example, U.S. PatentApplication Publication 2011/0108738, “Doped Luminescent Materials andParticle Discrimination Using Same,” which was filed by F.P. Doty et al.on Nov. 10, 2010 as U.S. patent application Ser. No. 12/943,708 and waspublished on May 12, 2011, describes scintillating materials that can beused to discriminate between energetic neutrons and gamma ray photons.The entirety of U.S. patent application Ser. No. 12/943,708, which isassigned to the assignee hereof, is hereby incorporated herein byreference.

As explained there, gamma photons tend more to create fast electrons inthe sensor medium, whereas neutrons tend more to create recoil protons.The scintillating materials reported by Doty et al. produce at least twotypes of luminescence which are distinguishable both by their temporaland spectral signatures. One type is fast luminescence associated withthe radiative decay of excited singlet states. The other type is delayedluminescence associated with a radiative mechanism for the decay ofexcited triplet states. The fast luminescence is preferentially excitedby fast electrons, whereas the delayed luminescence generally does notvary by particle type. Hence a preponderance of fast luminescence can betaken as an indication of gamma radiation as differentiated from neutronradiation.

The combination of stacked detector layers and particle-discriminatingscintillators can provide a high degree of discrimination amongparticles. That is, the less penetrating particles, i.e. alpha and betaparticles, and possibly soft x-rays, will substantially be detected onlyin the nearer sensor layer. Hence there will be discrimination betweentwo different classes of particle penetration which is independent ofdiscrimination between two different classes based on the type ofexcitation they provoke. Thus there are four possible combinations ofdetected results.

Photodiodes may usefully be employed as light detectors to detect thescintillation. If it is desired to discriminate by spectral signature,photodiodes may be made spectrally selective by, e.g., coating opticalbandpass filters onto their front (i.e., optical entry) surfaces. Thus,for example, in each of the two sensor layers in FIG. 10, onescintillator medium may be paired with a blue-selective photodiode, andthe other scintillator may be paired with a green-selective photodiode.

The two detectors in FIG. 10 are respectively a long-range detector 1010situated distal the radiation source and a short-range detector 1011situated proximal the radiation source. The long-range detector issimilar to the detector of FIG. 9. The short-range detector is modifiedfrom the design of FIG. 9 in that the cavity for the sensor medium isbrought closer to the surface proximal the incident radiation field. Asshown in the figure, strips of photosensor material, such asblue-sensitive and green-sensitive photosensor material, are optionallyprovided.

A “multimode” radiation detector combines at least two features of theradiation interaction in order to enhance performance over what isobtainable when only a single feature is detected. We will now describean embodiment that is “dual channel” or “dual mode” because the sensoris configured with the ability to detect both charge and light. Inexemplary implementations, detection of scintillation events is combinedwith with charge collection, in either ionization mode or proportionalmode. Generally, electron generation or charge-pair generation withinthe sensor material is a direct product of impingement by the radiationto be detected (although of course a secondary avalanche effect is alsoimportant for proportional-mode detection), whereas the scintillationlight that is detected is the product of a secondary interaction.

The radiation sensor, which includes a sensor medium that may be solid,gaseous, or even liquid, is mated to an Application Specific IntegratedCircuit (ASIC) 1100 that incorporates a horizontally integrated dualchannel pixel array 1110 as seen, for example, in FIG. 11. The figuredepicts a single macropixel comprising multiple channels of sub pixels1120, denominated “photon micropixels” or “optical sub-pixels”, that arephoton sensitive, and further comprising a single element 1130, denotedan “electron pixel”, for directly collecting charge.

Desirably, the integrated charge on each photon micropixel providesinformation indicative of photon intensity in a spatially resolvedmanner. The sub-pixel output signals can be summed off-chip, at themacropixel level, to provide a value of the total equivalent light seenby the macropixel. The respective sub-pixel output signals can also becollectively analyzed to obtain spatial information indicating thelocation within the macropixel that was impinged by the detectedphotons. Our design for the photon micropixel is derived from afront-side illuminated CMOS camera design concept that will be familiarto those skilled in the art. The optical signal and the direct-chargesignal will typically be processed separately, at least at the firstprocessing level.

FIG. 11 provides an example of a two-dimensional array. Additionally, webelieve that 3D arrays are achievable by stacking a plurality of sucharrays in a vertical column according, for example, to methods we havedescribed above.

We believe that a dual mode or multimode detector as described here willhave useful applications in medical imaging, industrial healthmonitoring, and national security, among other fields. It will beespecially useful for real-time event identification and tracking. Inparticular, we believe such a detector can be given the ability todistinguish whether a given interaction is due to a photoelectric event,pair-production, or Compton scattering.

The implementation depicted in top-down plan view in FIG. 11 employsfront-side illuminated photodiodes, as noted above. We believe that withappropriate changes in the fabrication process, a backside-illuminatedimplementation would also be possible. Such an implementation mightoffer the advantage of a higher fill factor for the photonic pixels. Inany event, we believe that a front-side illuminated system having anoptical pixel fill factor of 80% or even more is achievable.

As seen in the figure, the electron pixel, which is insensitive tophotons, is interleaved with the photon sensitive sub-pixels. In thefigure view, the observer's line of sight is directed downward towardthe face of the charge-collection electrode with which the electronpixel contacts the sensing medium. Charge-readout electronic circuitryis laid out on a pitch to fit under this electrode. The charge-readoutcircuitry may comprise, e.g., a conventional radiation sensor readoutelectronics chain, including charge-sensitive amplifier (CSA), shaperamplifier, peak detector, discriminator, and the like. Alternatively,the circuitry may comprise an integrating pixel design adapted toprovide image information.

Also seen in the figure are peripheral areas 1140 dedicated to supportcircuitry for the photon pixels, and similar areas 1150 dedicated tosupport circuitry for the electron pixel. Also seen is an outerperipheral region bearing an array of bondpads 1160, such asconventional 100-μm electrical bondpads.

FIG. 12 provides a cross-sectional view of a version of the sensordevice of FIG. 11 that has been adapted for a gaseous sensor medium. Asseen in the figure, this arrangement includes one or more insulatingspacers 1200 that separate a top plate 1210, which serves as, e.g., acathodic sensor electrode from a CMOS substrate 1220 and as such isenergized with a bias voltage. As further seen in the figure, the anodicelectrode is provided by a large pitch gold solder ball 1230 depositedon a top level bond pad of the CMOS substrate and is typically held atground level. The cavity 1240 defined between the cathode plate and theCMOS substrate serves as a pressure vessel filled with a gaseous sensormedium such as xenon or helium-4.

Typically, the sensor gas layer will be primarily insulating and hencecan cover the bondpads. The application-specific integrated circuit(ASIC) portion 1250 of the device of FIG. 12, including the front-sideilluminated photodiodes, the supporting circuitry for the photodiodesand the charge collection, and the gold anode can be made by standardSOI CMOS processes and known extensions thereof. Consistent withstandard SOI practices, the device shown in the figure includes a CMOShandle wafer 1260 overlain by a buried oxide (BOX) layer 1270.

FIG. 12 also schematically depicts a detector event caused by thecollision of a particle of radiation in the sensor medium. As shown, thecollision produces a shower of photons (represented by the symbol hν inthe figure), some of which impinge on, and are detected by, thephotodiodes 1290. The collision also produces a shower of electrons,some of which are collected by the gold anode.

FIG. 13 provides a cross-sectional view of a version of the sensordevice of FIG. 10 that has been adapted for a solid sensor medium suchas cadmium telluride or cadmium zinc telluride, which are very sensitivemedia for detecting x-radiation and gamma radiation by direct chargegeneration, and which have a very favorable electronic mobility-lifetimeproduct, which leads to long path lengths in charge-carrier transport.As seen in the figure, this arrangement includes a block 1300 of sensormaterial integrated with the ASIC substrate 1310 by flip chip assembly.

Fabrication of the device of FIG. 13 involves certain proceduraldifferences from fabrication of the device of FIG. 12 because the solidsensor medium of FIG. 13 obviates any need for a spacer of the kindused, e.g., to help define the gas cavity of FIG. 12. Because of thedifferent manufacturing methods, the bondpad structures 1320 of thedevice of FIG. 13 need to be exposed for proper bonding, as shown, andtheir placement may need to be adapted for such purpose. In addition, anoptically transparent underfill 1330 is required to reduce stresses andto facilitate mechanical alignment.

When operating in an imaging mode, the device circuitry will implementan integrating signal detector that accumulates signal energy forspecific windows of time. The photonic signal, which consists ofamplitude and spatial information integrated over respective such timewindows, is read off-chip. In parallel, the charge signal is integratedover the same time windows and read off-chip on a separate channel. Thecharge signal will provide amplitude information only.

In typical detection events, the generated photons are emitted into afull sphere. For greater collection efficiency, it is thereforeadvantageous to add a second detector ASIC 1400, symmetrically placedrelative to the first detector ASIC 1410, with the respective sensormedia 1420, 1430 proximal one another. Such an arrangement is providedin FIG. 14.

As noted above, on-board circuitry is advantageously included on thedetector substrates, including at least a preamplifier for each pixelor, in some implementations, for each group of pixels. FIG. 15 shows oneexample of such a dedicated circuit in high-level schematicrepresentation. As seen in the figure, the exemplary circuit includes apreamplifer 1500 connected to sensor 1505, a pulse-shaping amplifier1510, and according to various options may include any of an analogcomparator 1520, an analog peak detector 1530, and an analog-to-digitalconverter 1540.

What is claimed:
 1. Radiation detection apparatus, comprising: awafer-like substrate; a sensor medium; at least one optical sensordisposed on a surface of the wafer-like substrate containing integratedcircuitry, said surface herein denominated an active surface, said atleast one optical sensor optically coupled to the sensor medium so as tobe impinged by photons generated by radiation interactions within thesensor medium; at least one electric charge-collection electrode incontact with the sensor medium so as to receive charge carriersgenerated by radiation interactions within the sensor medium; and anelectronic signal-processing circuit electrically connected to the atleast one optical sensor and to the at least one charge-collectionelectrode so as to receive a photon-generated input from the at leastone optical sensor and so as to receive a carrier-generated input fromthe at least one charge-collection electrode; wherein the electronicsignal-processing circuit includes at least one preamplifier integratedon the active surface of the wafer-like substrate; and wherein theelectronic signal-processing circuit is configured to jointly processpulses due to the photon-generated input and pulses due to thecarrier-generated input.
 2. The apparatus of claim 1, wherein at leastone said optical sensor is a photodiode.
 3. The apparatus of claim 1,further comprising a wireless data-transfer port formed on thesubstrate.
 4. The apparatus of claim 1, comprising a rectangular arrayof optical sensors fixed in an integral unit with the sensor medium, andwherein said integral unit is one of two or more similar units arrangedin a vertical stack.
 5. The apparatus of claim 4, further comprising atleast one motherboard containing processing circuitry and connected toone or more output ports on each of the respective integral units. 6.The apparatus of claim 5, wherein each said integral unit includes anelectronic circuit configured to provide signal output that comprisestwo-dimensional image information, and wherein at least one motherboardincludes an electronic circuit configured to process the two-dimensionalinformation from the integral units so as to produce output thatcomprises three-dimensional image information.
 7. The apparatus of claim1, further comprising at least one further sensor medium fixed relativeto the substrate, wherein the further sensor medium is optically coupledto the at least one optical sensor and is separated from the at leastone optical sensor by a greater distance than the first sensor medium.8. The apparatus of claim 7, wherein the first sensor medium and the atleast one further sensor medium are disposed in a stack of layers suchthat at least some impinging radioactive particles can penetrate one ormore preceding layers before stopping in a layer of the stack.
 9. Theapparatus of claim 8, wherein the electronic signal-processing circuitis configured to discriminate radioactive particles and/or todiscriminate radioactive particle energies based on the number ofsensor-medium layers that are penetrated.
 10. The apparatus of claim 9,wherein two or more optical sensors are configured to respond todifferent products of interactions between the sensor medium andimpinging radiation, and wherein the electronic signal-processingcircuit is configured to further discriminate radioactive particlesbased on differences in the interaction products.
 11. The apparatus ofclaim 1, wherein at least a portion of the sensor medium is adjacent tothe active surface of the wafer-like substrate.
 12. The apparatus ofclaim 11, wherein the sensor medium is a gas or a liquefied gas.
 13. Theapparatus of claim 12, wherein the sensor medium comprises a pocket ofgas or of a liquefied gas encapsulated between the wafer-like substrateand a cover.
 14. The apparatus of claim 13, wherein the sensor medium isdisposed as a plurality of pockets of gas or of a liquefied gas, whereineach said pocket is encapsulated between the wafer-like substrate and acover, and wherein each said pocket is optically coupled to a respectiveat least one optical sensor and in contact with a respective at leastone electric charge-collection electrode.
 15. The apparatus of claim 12,wherein the sensor medium comprises at least one material from the groupconsisting of hydrogen, deuterium, helium, krypton, xenon, propane, andmethane.
 16. The apparatus of claim 11, wherein the sensor medium is asolid.
 17. The apparatus of claim 16, wherein the sensor mediumcomprises a semiconductor.
 18. The apparatus of claim 17, wherein thesensor medium is disposed as a plurality of semiconductor bodiessituated adjacent to the active surface of the wafer-like substrate, andwherein each said semiconductor body is optically coupled to arespective at least one optical sensor and in contact with a respectiveat least one electric charge-collection electrode.
 19. The apparatus ofclaim 16, wherein the sensor medium comprises at least one material fromthe group consisting of boron, cadmium telluride, cadmium zinctelluride, thorium oxide, uranium oxide, and thallium bromide.